Three murine plasmacytomas have been found which simultaneously secrete two classes of immunoglobulin. SAMM 368 secretes IgA and IgG2b paraproteins which do not share idiotype. TEPC 609 secretes IgA and IgG2b paraproteins which share idiotype. Both these tumors are from BALB/c mice. A third tumor, 2834, from an NZB mouse secretes IgA and IgG3 paraproteins; studies of the products of this tumor line are in progress.